Eddy current inspection is commonly used for non-destructive inspection purposes to detect flaws in surfaces of manufactured goods fabricated of a conductive material, such as bars, tubes, and other components for many industries such as automotive, aeronautic and energy, etc. Eddy current probes can be used with many configurations and/or inducing-sensing mode.
One of the most widely used EC probe configurations is the transmit-receive configuration. This configuration typically includes a driver coil and a receiver coil. For this configuraiton, the EC probe's driver coil is provided with alternative electrical excitation signals which generates an alternating electromagnetic field, resulting in a magnetic field on a test surface of a test object as the probe is moved along a path above the test surface. The magnetic field induces eddy currents on and near the surface of the test object fabricated of a conductive material. The eddy currents result in an electromagnetic signal or response on and near the test surface, which is received by the receiver coil and further analysed by an EC instrument/system As the eddy current probe passes over an anomaly, a flaw or a discontinuity in the test object, the anomaly disrupts the eddy current and results in a received signal that is irregular or out-of-pattern in comparison to signals received from normal objects. The deviation in the signal is used to indicate the anomaly. The alteration of the reading of the receiver is detected by the acquisition and analysis unit.
Typically, transmit-receive eddy current probes presents “directional sensitivity”, which means those EC probes are only effective to detect flaws that are approximately aligned with the “sensitive direction”, which is the driver-receiver axis. However, many test objects include complex surfaces, some resulting from complex manufacturing processes which create complicated stress patterns. Stress regions are prone to develop flaws which are randomly oriented, not necessarily being aligned with the directional sensitivity of the probe.
If the orientation of the flaw differs slightly from the sensitive direction of the probe, the result is a detection of the flaw with a sensitivity lower than that from the same flaw which is aligned with the probe. For example, a transmit-receive probe dedicated to 0 degree flaw detection, could be able to detect flaws at +/−20 deg but with a significantly smaller signal amplitude at the outer-boundary of the +/−20 deg. As a result, a shallow flaw perfectly aligned with the most sensitive orientation of the probe could be detected with a signal stronger than a deep flaw with an orientation slightly different from the directional sensitivity of the probe.
Usually the gravity of a flaw is defined by its estimated depth. As widely practiced, the decision to accept or reject an object being tested depends on the estimated depth of the flaw. If a reading of a flawing is larger than the acceptable limit, the object is rejected. However, if more than one flaw direction is anticipated and the reading of the flaw marginally meets or is within the acceptable limit, the user has to decide 1) to accept an otherwise rejectable test object, since the reading does not show the real depth of the flaw due the mis-alignment of the flaw with the sensitive direction, or 2) reject an otherwise acceptable object assuming the true depth is larger than it is measured. A third option is to repeatedly scan the component to detect flaws in different orientations, but the repeated scanning makes this process laborious and time consuming.
A special type of transmit-receive eddy current probe having two directional sensitivity, is known as an “orthogonal probe”. It is built with two coils wound orthogonally to each other on a non-conductive cube or a cross-shaped core. One of the coils is a “driver”, which is used to induce eddy current on the test surface, and the other is a “receiver”, which is used to sense the induced eddy current on the test surface. An important aspect of the orthogonal probe is that the fact that the driver and the receiver being perpendicular to each other decouples the driver magnetic field from the receiver, thereby reducing the sensitivity of the receiver to surface noise that does not represent a flaw.
U.S. Pat. No. 3,495,166 is incorporated by reference as the example for background art pertaining to the orthogonal EC probe herein mentioned.
In accordance with an important feature of the background art, the orthogonal EC probe includes field-sensing means for sensing fields produced by EC's in two regions having substantially the same spatial relation to a surface of the part and having a substantial angle therebetween with detector means being provided for detecting differences between the fields produced in the two regions. It should be noted that the sensing regions of the field-sensing means are orthogonal to the emitted magnetic field regions of field-producing means. Accordingly, in the absence of a defect that will disrupt the direction of the EC flow imparted by the field-producing means, the magnetic field resulting from the EC flow will also be orthogonal to the field-sensing means and will consequently not be sensed. With this arrangement, a high degree of sensitivity is obtained with respect to flaws having different orientations with respect to the sensing regions, while being insensitive to changes in a) conductivity, b) permeability, c) irregular surface finishes and d) to changes in the spacing of the part. This insensitivity stems from the fact that properties a, b, c and d affect predominantly the magnitude of the EC flow and resulting magnetic field, but not the direction.
As can be seen that orthogonal probes used with different inducing mode can therefore provide inspection to flaw orientations 0 deg, +/−45 deg, +/−90 deg, and with desirable insensitivity to noises such as conductivity, permeability, etc. However, this design still falls short of providing scanning sensitivity to flaws with any flaw orientation around 360 deg.
The inability or the inaccuracy to detect flaws in all orientation hinders any eddy current system from achieving higher performance and causes safety concerns. Repeating runs of scans falls short of being less efficient and cost effective.